void main(){
  var a;//未赋值的变量默认值为null
  print(a);
  a = 20;
  print(a);

  a="String>>>";
  print(a);

  var b=10;
  print(b);

  final c = '不能被修改的值'; 
  print(c);

  const d ="常量";//常量
  print(d);
}